1. Field of the Invention
The present invention relates to a non-aqueous electrolytic secondary battery comprising a non-aqueous electrolyte, a negative electrode that contains a carbon material, and a positive electrode that has a positive electrode current collector and a positive electrode active material-containing layer formed on a surface of the positive electrode current collector, the positive electrode active material-containing layer comprising a positive electrode active material containing lithium iron phosphate and a conductive agent.
2. Description of Related Art
Rapid advancements in size and weight reductions of mobile information terminal devices such as mobile telephones, notebook computers, and PDAs in recent years have created demands for higher capacity batteries as driving power sources for the devices. Non-aqueous electrolyte secondary batteries, such as represented by lithium-ion secondary batteries, use a non-aqueous electrolyte, and perform charge and discharge operations by transmitting lithium ions between the positive and negative electrodes. With their high energy density and high capacity, non-aqueous electrolyte secondary batteries have been widely used as the driving power sources for the mobile information terminal devices.
The non-aqueous electrolyte secondary batteries generally used have a positive electrode composed of LiCoO2, a negative electrode composed of metallic lithium, a lithium alloy, or a carbon material that is capable of intercalating and deintercalating lithium, and a non-aqueous electrolyte in which an electrolyte composed of lithium salt such as LiBF4 or LiPF6 is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate.
The use of Co, however, leads to high manufacturing costs because Co is an exhaustible and scarce natural resource. Moreover, the battery employing LiCoO2 has the problem of poor thermal stability when the battery in a charged stage undergoes an unexpectedly high temperature that would not reach in normal use.
For these reasons, utilization of LiMn2O4 and LiNiO2 as an alternative positive electrode material to LiCoO2 has been researched. However, the use of LiMn2O4 is not expected to achieve sufficient discharge capacity, and moreover, has such a problem as dissolution of manganese at a high battery temperature. Likewise, the use of LiNiO2 may cause problems such as low discharge voltage.
In view of these problems, olivine-type lithium phosphates such as LiFePO4 have attracted attention in recent years as alternative positive electrode materials to LiCoO2.
The olivine-type lithium phosphates are lithium composite compounds represented by the general formula LiMPO4 (where M is at least one element selected from the group consisting of Co, Ni, Mn, and Fe), and result in varied working voltages depending on the type of the metal element M. This leads to the advantage that the battery voltage can be freely selected by selecting the element M. Moreover, a large battery capacity per unit mass is achieved because the theoretical capacity is relatively high, from about 140 mAh/g to 170 mAh/g. Furthermore, for the element M, it is possible to use iron, which is readily available and low in cost, so the manufacturing cost of batteries can significantly reduce. Thus, the olivine-type phosphates are suitable as the positive electrode materials for large-sized batteries and high-power batteries.
Nevertheless, problems have still remained unsolved with the use of lithium iron phosphate as a positive electrode active material for non-aqueous electrolyte secondary batteries. Amine et al. report that when a battery comprising LiFePO4 as the positive electrode active material, graphite as the negative electrode, and an electrolyte in which LiPF6 is dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) is subjected to a cycle test at 55° C., dissolution of the iron from the positive electrode active material occurs, considerably degrading the capacity (see K. Amine et al. Electrochemistry Communications 7 (2005) pp. 669-673.). Such considerable capacity degradation at high temperature is a serious problem in large-sized batteries and high-power batteries, which are usually charged and discharged at a large current and are therefore likely to undergo a high battery temperature.
In order to solve the problem, Amine et al. have reported in the publication that the cycle performance at 55° C. improves by the use of an electrolytic salt LiB(C2O4)2 [lithium bis-(oxalato)borate] in place of LiPF6.
The just-mentioned LiB(C2O4)2 is, however, difficult to synthesize, and the use of LiB(C2O4)2 as an electrolytic salt is likely to increase the manufacturing cost of the battery. In addition, with lithium iron phosphate, the intercalation and deintercalation reactions during charge and discharge of the battery are slow, and in addition, the electron conductivity is much lower than those of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, and the like. For this reason, in the battery using lithium iron phosphate, resistance overvoltage and activation overvoltage increase particularly during high-rate discharge, which significantly degrades battery performance. In order to resolve this problem, it has been proposed to use a positive electrode current collector having a mean surface roughness Ra of greater than 0.026, to control the BET specific surface area of the conductive agent to be 15 m2/g, and to control the filling density of the positive electrode active material-containing layer to be 1.7 g/cm3 or greater (see WO2005/086260).
Although the above-described technique can improve high-rate discharge characteristics, it has the problem of poor cycle performance at high temperature.